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  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
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  <body>
    <script>
      var a1 = prompt('输入成绩');
      if (a1 >= 90) {
        alert('A');
      } else if (a1 >= 80) {
        alert('B');
      } else if (a1 >= 70) {
        alert('C');
      } else if (a1 >= 60) {
        alert('D');
      } else if (a1 <= 60) {
        alert('E');
      }
    </script>
  </body>
</html>
